Electric generating apparatus



H, M. HOBART ELECTRIC GENERATING APPARATUS Aug. 1s, 1925. 1,550,166

Filed March 1'7, 1922 Fjgl.

C Irn/entowI Patented Aug. 1%, i925.

stirrer s'rATEs A HERE? lfi. HOBART, SCHENECTADY, NEW YORE. AS$IGNGE TOGENERAL ELECTRIC CGMPAHY, A. CORPORATION OF NEW YORK.

ELECTRIC QENERATING APRQRATUS.

Application filed March 17, 1822. Serial Ho. 544,552.

To (zZZ whom it may concern:

a citizen of the United States, residing at Schenectady, in the countyof Schenectady, State of New York, have invented certain 'new and usefulImprovements iuElectrio Generating Apparatus, of which the following ise specification.

My invention relates to machinery for producing direct currentelectricity. More specifically, my invention relates to pro: vidingmachinery whereby the mechanical energy delivered from ahigh speed steamturbine or other high speed motor may be transformed into direct currentelectricity with less loss of energy incurred in the transformation andwith less outlay for 'upkeep and attendance'than it is possible toobtain when this transformation is effected,

by other means at present,- aveilable. These objects-are attained in ahigher degree the higher the voltage ofltheydired current electricity.My invention is thus crating apparatus for the so-cslled Thurytransmission system. In carrying my 1n rention into eii'ect, I make useoi one or \ore vapor rectifiers supplied from a high rectifiers' whenconnected in series or parel lel whereby the frequency of the pulsationsin the direct current voltage are increased and their amplitude reducedto a minimum;

Uther adventages of my invention will appear as the descriptionproceeds.

Hcretofore, three fundamental types of apparatus have been used fortransforming mechanical energy, delivered from steam turbines, intodirect current electricity. Thev first and earliest type comprises anordinery direct current generator connected to the shaft of the steamturbine. The mechanical und electrical limitations to the dcsignol thecomuuitator were, however, so restricted that was necessary to designthe steam turbine for only a fraction of the speed corresponding to thelow steam consumption per horse power hour of output from the stesi'ntru'bine shaft which artic'u larly advantageous when used as he gen,-

obtainable with designs for higher speeds. Such outfits were neversuccessfully developed for speeds suii'iciently high to obtain thebest-results from steam turbines or for other than relatiiely veryslnalldirect current outputs.

The second type of direct current apparatus known to the prior artcomprises a fairly high speed steam turbine delivering mechanical energythrough speed reducing gearing to medium speed direct currentgenerators. By this means it is practicable to employ much higher speedsfor the steam turbine than in the first type of apparatus above referredto. A speed of 5,000 revolutions per minute is often employed for theturbine, which is still below the most economical speed as regards steamconsumption.-, Speeois of the order of 1,000 revolutions per minute forthe direct current gen erator have been used, which are still higherthan desirable for the satisfactory commutation of heavy currents-of anyconsiderable voltage. The speed reducing gearing isexpensive to installand maintain; it is noisy, and is the seat offriction losses rangingfrom 2% and more at rated load u to and more at quarter load.

The third type of direct current generating apparatus known to the priorart was devised by the present applicant in 1902 and has been used in afew installations. It is described on page 403 of Hobart and Ellis -HighSpeed Dynamo Electric Machinery,

and consists in a polyphase generator di rect connected to a high speedsteam turbine, and a synchronous converter supplied with polyphaseelectricity from the stator windmg of the polyphase generator. Hereagain, however, one (it; the other of the sets must be inefficientlydesigned and operated because the steam turbine is essentially a highspeed machine and the synchronous converter a comparatively low speed,low frequency machine.

"The voltage obtainable from a direct cur rent commutator is limited,especially where it is called upon to deliver heavy currents.

It has not been found practicable to go above about 2500 "olts percommutator for currents above 1000 amperes. Thus, t'ora 1000 ampere200,000 volt Thury system we would require direc current connnutetorsconwindings 4 and 5.

nected in series. Although the transmis sion economies claimed for anddoubtless attainable by the Thury system are very attractive,nevertheless this system is not favored because heretofore the proposedgenerating equipment was too expensive and uneconomical. For this reasonit has never been adopted on a sutficiently large scale to takeadvantageof its great economies. My invention provides a much more economicalsystem of generating. high voltage direct current suitable for the longdistance transmission of power by using high frequency alternatorssupplying vapor rectifiers connected in series.

The features of my invention which I believe to be novel and patentablewill be pointed out in the claims appended hereto. The construction andmode of operation of my invention will be understood from the followingdescription taken in connection with the accompanying drawing, in whichFig. 1 represents a high speed turbo alternator having a plurality ofseparate alterating current windings, each connected with a vaporrectifier and supplying a direct current system; Fig. 2- is a diagrammatic modification of my invention suitable for supplying a high voltagedirect current transmission system: and Fig. 3, shows curvesillustrative of the voltage supplied by a pair of rectifiers whenconnected in accordance with one feature of my invention.

Reterring now to Fig. 1, 1 represents a steam turbine designed for ashigh a speed as is consistent with reliability and low steam consumptionper horse power hour of output. 2 represents a rotating field of analternating current generator directly connected to the shaft of thesteam turbine. The rotor 2 is excited by direct current through suitablebrushes and collector rings from a source The stator of the alternatingcurrent generator is provided in the present example with a pair ofpolyphase These polyphasc wind ings are connected in star, or areotherwise connected so as to provide a neutral point for each winding. Ihave represented the two windings 4; and 5 as having the same number ofphases. This however is not essential as will appear later. hen thephases of the two or more sets of windings are equal in number, theywill preferably be displaced with respect to each other asdiagrammatically illustrated. Thus, the upper phase of winding 4, isshown in the vertical position, while the upper phase of winding 5stands at an angle to the vertical line. Where two sets of 5 phasewindings are employed they will preferably be displaced 36 with respectto each other so as to make 10 equally spaced phases on the machine. I

erator to tlrc'rcti er as short'as'practicable.

Reeti fiersj daiid" 9 eaclfhave arcath'ode e10 located equally distantfrom the anodes and; serving as a cathode for all of them. The polyphasetype of vapor rectifier such as is here diagrammatically illustrated iswell known and need not be further described.

l to'willi now be evident that when the generator is in operation, adirect curent may be drawn ott from the cathode'lO and the neutral point6 of either or both systems, one system constituting generator winding 4and rectifier 8, and the other system constituting generator winding 5and.

rectifier 9. Thus winding 4 will generate a synchronously rotating.polyphase electromotive force at the anodes 7 of rectifier 8,

and an arc discharge will flow between the cathode and the anode oranodes which are positive with respect to this cathode when consumingdevices are connected between the neutral point 6 and the cathode 10.The electron stream established between the cathode and the anodes maybe likened to a brush rotating synchronously with one end attavheo tothe cathode. A. similar operation will take. place in rectifier 9receiving polyphasc energy froinwinding 5. The two systems may be usedseparately, or, if the two systems are designed for equal voltage andcurrent carrying capacity. they may be connected in series or parallelto supply a direct current circuit. The two systems are illustrated asconnected in series to supply a directv current circuit 11. Thus,cathode 10 of rectifier S is connected through line 12 and switch 13 tothe neutral point 6 of alternating current winding 5, while neutralpoint 6 of winding 4 and the cathode 10 ot' rectifier t) constitute theterminals of direct. current circuit 1l.' I also show a switchingarrangement whereby the two systems may be connected in multiple tosupply circuit 11. For this purpose switch 13 is thrown down to make connection with contact 14, which will connect the two neutral points 6through line 15 to one side of circuit 11 and switch 16 will be closedconnecting the two cathodes 10 together to the other side of circult ll.

Referring now to Figure 3, a direct current voltage produced by the twosystems may be represented by the exaggerated curves A and B,respectively. The pulsating character of the voltages represented bycurves A and B is due to the'shifting of one end of the electron streamin the rectifiers from anode t0 anode. Since the alternet- I sultantdirect current voltage will be steading current windings 4 -andv 5 aredisplaced withrespect to each other, the main. portion of the electronstream in one rectifier will be shifting between two consecutive anodesat the instant the main part of the electron stream in the otherrectifier is directly on an anode. In this manner the pulsations in thedirect current voltage produced by the two systems are madeto overlapand' if they are connected in series or parallel the reier than" wouldbe thecase if the two alterthese two windings.

windings in suitable mechanical positions nator windings were notdisplaced. Thus in Figure 3 curve C may represent the resultant directcurrent voltage when the two systems are connected in series. To furthereliminate the pulsations of the direct current supplied due to thepulsating character of the voltages I preferably include a small reactor17 in one of the outgoing lines of the direct current circuit 11. ItWlll be obvious that any number of such systems connected in series orparallel or in series parallel may be supplied from the same generator,and

ofthe direct current voltage fluctuations will be the same intwo'rectifiers supplied from By locating the two with relation to oneanother, the pulsations will be as represented by A and B in Fig. 3 andthe resultant will be as represented by C in Fig. Y

I may if desired have windings on the same generator for difierentvoltages permitting the same to simultaneously supply, throughrectifiers, direct current circuits of different voltages. 'It shouldalso be observed that the voltage of the direct current circuit orcircuits may readily be controlled by varying the excitation of thealternating current generator. Furthermore my invention is notrestricted to the case of steam turbine drive, but itsadvantages willoften make it desirableto employ it for obtaining direct currentelectricity from a source of polyphase electricity. In such a case avery iigh speed synchronous motor operated from a polyphase supply isdirect connected to the polyphase generator which in turn supplies themercury arc rectifier or rectifi'ers. I consider that for manyconditions this arrangement is superior tothe-usual arrangements, whichare as follows:

(1-) Moderate speed synchronous motor direct connected to adirectcurrent generator.

(2) Static transformers supplying a synchronous converter.

' 3(3) Static transformers supplying a mercury arc rectifier.

The number of poles for which the rotor 2 of the alternating currentgenerator is wound is determined from economic con siderations andpracticable proportions, but so far as such considerationsare found tobe substantially equal for two different number of poles the largestnumber of poles will be used, since it is desirable in my invention,particularly where few rectificrs are connected with overlappingpulsating voltages, to employ the highest economically practicablefrequency. The higher the frequency the more completely will any.pulsations in the rectifier current he'smoothed out by a reactor ofreasonable proportions. As of still greater importance it should benoted tliatthe higher the frequency of the currents. supplied to therectifier the less need be the number of anodes for providing directcurrent with a given degree (if freedom from pulsations. The smaller thenumber of anodes in a mercury arc rectifier the smaller need be the sizeof the vessel in which the -auodesare contained and the more readily cana high vacuum be maintained in this smaller vessel. 01', if it is .notnecessary to strive to decrease the size of the vassel, it may be saidthat 'the smaller-the num ber of anodes the less will be the leakage atthe anode seals and the more liberal may be the lateral distance betweenneighboring anodes and the more simple will he -the design of thecooling systems. On the other -105 band, it is true that anode heatingisa limitation to the output of a'rectifier andconsequen'tly a'reasonable multiplicity of anodes distributes the total current amongsta reasonably large number of anodes and in' creases the facility withwhich; it can be arranged to rapidly convey heat away from theseanodes.Six anodes associated with a to be quite good enough (with respect tothe freedom from pulsations in the direct current circuit) for importantservices and the tendency is' to arrange to employ twelve anodes. In oneof the most important installations to which rectifiers are now beingemployed, these twelve anodes aresubdl- 5O cycle supply is at presentconsidered, not

vided in two vessels, there being six anodes in each vessel. By means ofthe provisions of the present invention equally-good results in thisrespect will be obtained with only six anodes and a 100 cycle supply andmuch better results with six anodes and a 200 cycle supply. 4 v

To give quantitative form to the propos tion, let us assume that thesteam turb ne is designed for 6000 revolutions per minute and thepolyphase generator for four poles: This combination would provide a 200cycle polyphase supply and We might even increase the number of poles onthe alternating current generator to 6, giving a supply of 300 cyclespcr second. Let, us assume that a direct current voltage of 1500 voltsis desired It is well known that for this voltage the rectifier willhave an eiliciency of about 98.5 per cent. At present few, if any,mercury arc rcctifiers are in service for delivering continuouslycurrents of over 600 ampcres per vessel, although they will carry verymuch larger "currents for short times. The normal rating of such arectifier in a single vessel would be 1.5 600:900 kw. But let us assumethat we employ four such rectifiers in parallel and suitably locatedadjacent to the alternating current. generator. Then. the total ratedoutput 'Will be 3600 kw. lhe efiiciency of such a generator, includingall the losses in it would be of the order of 96 per cent. one per centfor reactors and for any other auxiliaries such as vacuum pumps andWater circulating pumps, We obtain a total full load eliiciency of 93.5per cent from the shaft of the steam turbine to the direct current cirvcuit. No such high elilciency as this can be obtained With any of thethree earlier methods, which I. have described above. At half andquarter load the advantage will be still more overwhelmingly in favor ofthis p'ropov .sitlon. Moreover, there is the further gain that the steamturbine has a lower steam consumption per H. P. hour of output thanwould be practicable by any of the other three schemes. For theparticular case e1nployed for purposes of illustration, the nor mayoutput of the steam turbine will be about 5000 H. P.

It is a fair analogy to describe my out-lit as derived from the ordinarydirect current generator by (1) substituting an open cir? cuit Windingfor the closed circuit minding.

used on ordinary direct current generators; (2) the open circuit windingis located on the external stator and the direct current excil'cdfieldon an internal. rotor, thus reversing the arrangement employed on anordinary direct current generator; (3) in place of a comn'iutator on ashaft with all the.

limitations imposed by limiting peripheral speed and practicable numberof segments and practicable speeds at which current can be collected bycarbon brushes resting on the surface, and limiting voltages betweensegments, I substitute the mercury arc rectifiers stationary anodes forthe conunutator segments and am thus free from all questions associatedwith centrifugal force and high peripheral speed; (i) for the carbonbrushes and bruslrholders with the associated problems of collection andmomentum Let us allow a further loss of single rectifier supplied fromeach gene of the transformer Winding "in mechanical arrangements withbrushes there must be supplied means 1'01" driving the brushes atsynchronous speed, but the electron stream brush moves from segment tosegment at synchronous speed in response to the cyclic shifting of theinaXimum voltage and the cathode end of the brush does not becometwisted up by the ro-- tation of this gaseous brush- In Fig. 2, I haverepresented a modification of myinvention suitable tor ylying theso-called Thury transmission system. In this figure I show prime moverssuch as water or steam turbines 18, 19, and 21. These prime movers aredirect connected to alternating current generators 22, 23, T24: and 2-5,res ectively. These four generators are utilize to supply four vaporrectifier systerns represented at 2.6, 538 and 29,

I t 1..., I 1 spectively. Although 31 have shown on a it will beapparent that the latter in ply a plurality of 'recti connection withTi. that the middle generators connected directly to their, rectiii rsii a; 28, while the outer two generators an i are connected to theirrectifier-s E26 and through step-up, phase multiplying lira formers 30and 31 respectively. It v-ril :3 iso be noticed that the rectifier 529has if anodes, rcstifier 527 has five anodes. rectifier 26 has sixanodes and rectifier has seven anodes. the alternating current windingsfrom "which these recti'iiers are supplied he inn" provided with thecorresponding number of star connected phases. (-leneratoriiii is a 3phase star connected machine supply ing the 3 phase, .lPSll connectedprimary 312 of transformer 30, the secondary 13 of which is a starconnected 6 phase Winding supply ing the six anodes oi. rectifier E36;generator 23 has a 5 phase, star connected supplying the live anodes ofwet generator 24 has a 7 phase, star connected winding! subhlyingl theseven anodes of res tiller 2) and generator 25 is a 3 pha i. sit.connected generator supg'ilyingg the 2; i \ase, mesh connected primary34 n1 transbrmer 31. the secondary 35 of which is star corrnrcted withfour phases supplying the four anodes oi rectifier The Four connected inseries illustrated. The thode of rectifier 26 and the neutral. pointconstitute the terminals of the high voltage, direct currenttransmission line 11. The cathode oi rectifiers 27. 28 and 99 areconnected to the neutral points of windings 3Z1, and

' The rotating electron stream; discharges in respectively. By makingthe various rec--' tifiers with different number of anodes and providingthe alternatingcurrent winding to which they are connected withcorresponding different number of phases, I pro, vide another method ofincreasing the periodicity of the voltage pulsations of the rectifiers.Thus, assuming the frequencies produced by the four generators areequal. and that the generators are in synchronism with one another, itwill be apparent that the synchronously rotating gaseous brushes in eachrectifier. will rotate in synchronism. However, since the number ofanodes in the various rcctifiers are unequal, the voltage pulsations ofthe various-rectifiers will'overlap and the resultant pulsations will beof higher periodicity in the series-circuit and their amplitude may bemaintained low b a relatively small reactor. It will be obvious that itwill not'be ne'ce'ssary or even desirable to maintainexact synchronismbe-- tween the various generatorsto accomplish this result. Instead ofmaking the series rectifier systems of a different number of phases, Imay accomplish the same result by maintaining the phases equal andmaking the frequencies unequal. Thus I might provide each system' withsix phases and maintain frequencies of 180, 190, 200 and 210 cycles persecond on the four generators the various rectifiers would then be out01 synchronism with each other and the voltage pulsations would overlapthus estab' lishing higher frequency pulsations in the resultingvoltage. This feature of obtainingincrcased frequency of pulsations inthe direct current circult without the need for synchronous operation ofthe prime movers or motors driving generators, works in very nicely inthe case of a high-voltage transmission system spreadover. ,a large areasince it permits oi looping: into: the series connected to fouralternating cur-rent system rectifiers suppliccliromzlocal sourcesof'power in various parts ofthe scattered area. As a practicalillustration of the system. outlined in Fig. 2, let us: assume theproblem of generating 100,000'lrw. at 200,000 volts and 500 amperes fordirect current transmission. Let 18, 19, 20 and 21 represent four waterpower driven turbines direct onerators 22, 23, 24, and 25 such that thelat .ter each have a rating of 25,000 kw. Generators 23 and 2a will belocated on either side of the grounded point 36 and conse-= quently willhave a maximum difference of potential from earth of 50,000 volts interms ofdirect current voltage, whichit should be noted is no moresevere than 35,000 volts alternating current. This potential differenceis feasible if the stator windings are immersed in oil to prevent coronaefi'ects. The outer -would be connected in series-to rectifiers through.transformers 30 and 31.

Sincethe maximum. voltage to ground on these rectifiers is 100,000it isno lcngerdd sirable to go straight from the terminals of the generatorwindin s to the anodes of the rectifier-s. Although? one 'transformr,two as desired with each v outer generator. It will not be necessary'toimmerse the stator windings or" the outer generators in oil since Comingnow to the rectifiers, outfits for large currents at 1500. volts arealready articles of commerce and I will base this illusthat it isfeasible,

trative example on only 2000 volts per rc'ctlfier, although I considerby using rectifiers known as the steel mery cury arc type, to obtain5000 volts at 500 amperes from each rectifier. On the basis of 2000volts per rectifier each generator would supply twenty-five suchrectifiers all connected in series. These rcctifiers wouldv be mountedon insulated platforms and the total of 100 rcctifiers for the fourgenerators 200,000 volt direct current Thurypply transmis- '70 haverepresented only or more may be used they will be for only moderatevoltages.

sion system. With this. many rectifiers 0011* nected in series andarranged to have the voltage pulsations overlapped by the :use of one ormore of'the methods previously de-- scribed, the high frequencypulsationswill be smoothedout by a' relatively small reator such-asshownat 17.

The speed, number of poles and frequencies of the generators will begoverned more or less-by economy and. design and the. nature of thehydraulic conditions. However,

for-reasons heretofore stated, I prefer to use a fairly high frequency,especially for those generators supplying the rectifie'rs through theintermediary of transformers or compensators. The transformers 30 and31:may*be made smaller, cheaper and more efiiciently when highfrequencies are: used. The generators 19 and 20, not having trans-Vformers may if desirable, have somewhat lower rrequencie-s, especiallyif wound for more phases. Voltage control will preferably be maintainedby generator field adjustment either automatically or manually.

As to the efficiency of this generating system, the generators wouldhave/the usual st ficiency of generators of this rating, pre sumably,some 96%. 1F or the inner genera? tors there are no transformers and theefficiency of the rectifiers' would be 00.5%,

thus the combined efiiciency of the inner generators. andith'eirrectifierswould be, 94.5%.= Theoutergenerator systems must.

be debited-with the losses inthetransform eye; butt since. these are forhigh; frequencies;

their ratin hwingsu eficiency o--99%so*z that the combined efficiency ofthe outer generators, transformers, and rectificrs would be 93.5%. Letus allow a further one-half per cent for losses in various auxiliarymachinery. The total efficiency 'of the generating plant including allgenerators, transformers and rectifiers would, therefore,- be.

per cent. This is certainly considerably igher than would be possiblewithanygencrating proposition for the Thury system involving directcurrent generators. Furthermore, the capital outlay for such a systemwould be much below the capital outla for a Thury generating system ofthe type lieretofore contemplated .and employed.

.In certain of the appended claims, the

expressions high speed corresponding to,

maximum" economy and -a frequency as hlgh as 1s practicable consistentwith econ.-

. omy and reliability have been used. I have hereinbefore'given examplesof certain desirable combinations of values for the speed and frequency.It will be evident that the speed to be usedin any particular case willdepend upon the type of prime mover 'or' Anotor to be used and also itscapacity. -For 9 instance, the most desirable speed from the standpointof a medium sized steam turbine might be 7,000 B. P. M., whereas, itwould probably be more desirable to operate a large capacity waterturbine at a somewhat lower speed. The most economical speed of thewater turbine will also depend on the.

water pressure available, etc. The fre- .quency to be selected; shouldbe as high as is practicable, consistent with eopnomy and re liability.If the direct connected generator is to be driven at 7,000 B. P. M., itmay safely and economically be designed with 4 poles giving a frequencyof about 233xcycles per second. If a somewhat lower speed is used, itwill be desirable to use a greater number of poles on the generator,being.

careful to keep the peripheral speed of the rotor within the limits ofsafe mechanical design. With this explanation, it will be clear whatImean by the expressions above referred to. evident that I do not wishto limit the invention to any particular set of values since thesevalues depend upon a number of variable considerations and upon eachother.

In accordance with the provisions of the patent statutes, I havedescribed the principle of operation of my invention, together with theapparatus whichv I now consider to represent the best embodimentthereof, butI-desire to have it understood that the apparatusshown anddescribed is only illuswtrative and that the invention may e carried outby other means.

At the same time, it will'be sired high direct current voltage, agrounded connection for the series connection and transformers in thesupply connections of certain of the I 8Ctlfi81 systems whereby thepotential to ground on each generator is kept within safe limits.

2. An electric generatim system for producing high voltage directhurrents comprise ing a plurality of generating outfits, each generatingoutfit comprising a motor driven alternating current generator and aplurality of rectifying systems supplied thereby, said rectifyingsystems supplied from different enerators being of different phases andeac com rising a polyphase vapor rectifier 'suppli from a winding on itsrespective generator, means whereby various rectifier systems of all thedevices may be connected in series to obtain the desired high directcurrent voltage, a grounded connection for the series connection andtransformers inthe supply systems of certain of the recti ing systemswhereby the potential to groan on each generator is kept within safelimits. I

3. Anelectric generating system for producing high voltage directcurrents comprising a plurality. o -generating outfits, each generatingoutfit comprising a motor driven alternating current generator and aplurality of rectifying systems sutpplied thereby,

said generators being of di erent frequen-.. cies, each rectifyingsystem comprising a poly hase vapor rectifier supplied from a win ing onits respective generator, means whereby various rectifier systems of allthe devices ma the desire tion' and transformers in the supply systemsof certain of the rectifying systems whereby be connected in series toobtain high direct current voltage, a grounded connection for the seriesconnecthe potential to ground on each generator is keptwithin safelimits.

4. An-electric generating system for producing high voltage directcurrents comprising a plurality of generating outfits, each generatingoutfitcomprising a motor drivenalternating current generator, saidgenerators being of different frequencies and a plurality of rectifyingsystems supplied t creby, the rectifying systems supsystems of all thedevices. may be connected in series to obtain the desired high directcurrent voltage, a grounded connection for the series connection andtransformers in the supply systems of certain of the rectify- 1 ingsystems whereby the potential to ground on each generator is keptwithinsafe limits. .In Witness whereof, I have hereunto set my hand this16th day of March, 1922.

HENRY M. HOBART.

